Optical amplifying repeater

ABSTRACT

An optical amplifying repeater is provided which enables even after the main input optical signal is disconnected, the continued returning of a response signal in response to a command signal from an end station. This optical amplifying repeater comprises an optical amplifier, an automatic level control loop including therein a drive control unit, a supervisory unit which returns a response signal RS responding to a command signal CM from an end office, and a detection/control unit which controls the drive control unit so as to increase the amplitude of the response signal RS when a loss of the main optical signal S from an optical transmission line is detected.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical amplifying repeater whichforms the central part of, for example, a submarine optical amplifyingand repeating apparatus.

At present, in long-distance transmission of optical communicationssystems, the so-called optical amplifying and repeating systems arecoming into wide use, in which systems optical amplifiers are containedin the above repeaters.

In such optical communications systems, because the distance from oneend office to another is extremely long, it is not easy to monitor orcontrol each of the optical amplifying repeaters which are cascadeconnected between the two end offices.

For this reason, in general, to perform monitoring or control of each ofthe optical amplifying repeaters, a remote repeater monitor and controlsystem, which uses light modulation, is employed. The present inventionincludes an improvement to an optical amplifying repeater which ismanaged by a remote repeater monitor and control system.

2. Description of the Related Art

As will be described in detail with regard to the accompanying drawings,in a multi-stage optical amplifying and repeating transmission system, along-distance optical transmission line is laid between two end officesA and B, a plurality of optical amplifying repeaters being connected incascade along this optical transmission line.

Assume that a cable failure occurs when data is being transmitted fromend office B to end office A. When this occurs, because the normalreception of data at the end office A is suspended, end office Aimmediately starts a search for the causative fault. This fault searchis performed by the use of a command signal CM by which a main (carrier)optical signal S is modulated and the response signal RS which is outputfrom each amplifying repeater in response thereto.

In the above-noted system, when the above-noted cable failure occurs,the optical carrier signal S is no longer input to the opticalamplifying repeater. This is a loss of the optical input.

When this optical input loss occurs, because the optical carrier signalS is no longer input to the optical amplifying repeater, it becomesimpossible to apply the modulation of the above-noted response signal RSto this optical carrier signal S. When this occurs, this opticalamplifying repeater applies RS modulation instead to the ASE (amplifiedspontaneous emission). This ASE is a noise component of a relativelywide bandwidth which is inevitably generated by each optical amplifyingrepeater.

Although it is theoretically possible to apply modulation in accordancewith the response signal RS to this ASE, in actuality the modulation ofthe amplified spontaneous emission by the response signal RS isinsufficient. As will be described later, in the range in which thedrive current I_(LD) of the optical amplifying repeater is small, thevariation of the amplified spontaneous emission with respect to thedeviation in I_(LD) is relatively large.

In response to the loss of optical input, when the ALC (automatic levelcontrol) loop operates sufficiently, so that drive current I_(LD) Of theoptical amplifier becomes large, in this range in which I_(LD) is large,the variation in the output of the amplified spontaneous emission isrelatively small. For this reason, when applying modulation to the ASEwith the amplitude of the normal response signal RS, the resultingmodulation output is extremely small. Ultimately, after the opticalcarrier signal S is lost, even if modulation is applied to the ASE bythe response signal RS, it becomes difficult to perform such modulation.As a result, it becomes difficult to receive the response signal RS atthe end office and, in the worst case, it becomes impossible to monitorthe optical amplifying repeater, and impossible to performtroubleshooting of the fault point.

SUMMARY OF THE INVENTION

In view of the above problems, an object of the present invention is toenable the reception of the response signal at an end office, even afterloss of the optical carrier signal due to a cable failure.

To achieve the above-noted object, the present invention is configuredso as to comprise an optical amplifier, an automatic level control loopwhich includes a drive control unit, a supervisory unit which returns aresponse signal RS responding to a command signal CM from an end office,and a detection/control unit which controls the drive control unit sothat the amplitude of the response signal RS is increased when the lossof the optical carrier signal S received from the optical transmissionline is detected. In this manner, it is possible to continue returning aresponse signal in response to a command signal from an end station,even after the main input optical signal is disconnected.

BRIEF DESCRIPTION OF THE DRAWINGS

The above object and features of the present invention will be moreapparent from the following description of the preferred embodimentswith reference being made to the accompanying drawings, wherein:

FIG. 1 is a drawing which shows the first basic configuration of anoptical amplifying repeater according to the present invention;

FIG. 2 is a drawing which shows an example of the first configuration ofa detection/control unit 41 according to the present invention;

FIG. 3 is a spectrum plot of the amplified spontaneous emission (ASE)which exists along with the main optical signal (carrier);

FIG. 4 is a spectrum plot of the amplified spontaneous emission (ASE)when the main optical signal (carrier) is lost;

FIG. 5 is a drawing which shows an example of the second configurationof the detection/control unit 41 according to the present invention;

FIG. 6 is a drawing which shows an example of the third configuration ofthe detection/control unit 41 according to the present invention;

FIG. 7 is a drawing which shows the second basic configuration of anoptical amplifying repeater according to the present invention;

FIG. 8 is a drawing which shows an example of the first configuration ofa detection/modulation unit 61 according to the present invention;

FIG. 9 is a drawing which shows an example of the second configurationof a detection/modulation unit 61 according to the present invention;

FIG. 10 is a drawing which shows an example of the third configurationof a detection/modulation unit 61 according to the present invention;

FIG. 11 is a drawing (1) which shows the details of block 22 and block23 which are shown in FIGS. 1 and 7;

FIG. 12 is a drawing (2) which shows the details of block 22 and block23 which are shown in FIGS. 1 and 7;

FIG. 13 is a drawing which shows the general configuration of a usualmulti-stage optical amplifying and repeating transmission system;

FIG. 14 is a drawing which shows the configuration of a usual opticalamplifying repeater; and

FIG. 15 is a graph which shows the relationship between the drivecurrent and the amplified spontaneous emission (ASE) of an opticalamplifier.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Before describing the embodiments of the present invention, the relatedart and the disadvantages therein will be described, with referencebeing made to the related drawings.

FIG. 13 is a drawing which shows the general configuration of a generalmulti-stage optical amplifying and repeating transmission system. Inthis drawing, the reference numeral 10 denotes a multi-stage opticalamplifying and repeating transmission system, in which a long-distanceoptical transmission line 12 is laid between two end offices (A and B),a plurality of optical amplifying repeaters 11 being connected incascade along this optical transmission line 12.

The optical transmission line 12 normally comprises a pair of lines, anoptical amplifier (AMP) connected to one, which is for one direction,and to the other, which is for the other direction. Thus, transfer ofvarious data is performed via optical signals between the two endoffices.

If the above-noted multi-stage optical amplifying and repeatingtransmission system 10 is a submarine optical communications system, theoptical transmission line 12 would be of an extremely long length of,for example, several thousands of km. For this reason, if a failureoccurs along the optical transmission line 12, the failure is located byremote control from an end station. In addition to when failures occur,remote control is also performed from an end office when maintenance isdone. For this reason, monitoring or control using optical modulation isperformed from an end office via the optical transmission line 12 withrespect to each optical amplifying repeater 11 individually.

For example, assume that a failure occurs at the point marked "X" in thedrawing when the end office B is performing data transmission to endoffice A. If this occurs, because the normal reception of data that hadbeen performed at end office A is interrupted, the end office Aimmediately starts to search the offending failure. In searching forthis failure, a command signal CM which is modulated on the main opticalsignal (carrier) and a response signal RS which is output from eachoptical amplifying repeater 11 in response to the CM are used.

In the case of the above-noted example, the command signal CM is sentfrom the end office A. This CM command is normally is a signal ofseveral megahertz to several tens of megahertz which is superimposedonto the optical carrier signal. As an example, if a 10-MHz signal ispresent, the data is "1", and if the signal is not present, the data is"0". The above-noted response signal RS is usually a signal of severalkilohertz which is modulated onto the optical carrier signal, wherevarious information of the signal RS being represented by the particularfrequency thereof.

The optical amplifying repeater 11 which sequentially receives thecommand signal CM from the end office A generates a response signal RSin response to the instructions which are included in the command signalCM, this being returned to the end office A via a loopback LB which isformed within the optical amplifying repeater 11. The end office Ainterprets the returned response signal RS to determine the state ofeach optical amplifying repeater 11. In the example shown in thisdrawing, since there is a discontinuity which occurs in the RS signalsfrom the optical amplifying repeaters 11 which are immediately next toone another including the position "X", therebetween the end office Ajudges the failure to be located in the region of the point "X".

In the above description, while the example used is that in which afailure has occurred in the lower, left-directed, optical transmissionline 12 in the drawing, if a failure occurs in the upper, right-directedoptical transmission line 12, end office B would perform the same exactoperations as described above for the end office A. In doing this, thedirection of loopback LB would be the opposite of the arrow shown by thearrows.

FIG. 14 is a drawing which shows the configuration of a usual opticalamplifying repeater, which optical amplifying repeater 11, asillustrated, broadly speaking, comprises an optical amplifier 21, an ALC(automatic level control) loop 22 and a supervisory unit 23.

The optical amplifier 21 is inserted into the optical transmission line12, and amplifies the light which it receives.

The automatic level control (ALC) loop 22 includes therein a drivecontrol unit 24 for controllably driving the optical amplifier 21 andmaintains as constant the output light level from the optical amplifier21.

The supervisory unit 23 decodes instructions which are included in thecommand signal CM from an end office located at one end of the opticaltransmission line 12, and applies a response signal RS to the drivecontrol unit 24 in response to these decoded instructions.

The optical amplifier 21 comprises for example an EDF (erbium-dopedfiber) 31, a WDM (wavelength-division multiplexer) 32, and a laser diodeLD 33. While this drawing shows what is known as a backward-pumping typeof optical amplifier, it is also possible to make use of aforward-pumping type of optical amplifier.

Part of the light from the optical amplifier 21 is split off by means ofthe optical coupler 34. The thus split-off light output is firstconverted to an electrical signal by means of a photodetector (PD) 35,this being then applied to the automatic level control loop 22 and thesupervisory unit 23.

The automatic level control loop 22, in addition to a drive control unit24, has an automatic level control (ALC) circuit 36. This circuit 36detects the difference between a reference level which is set beforehandand the output light level from the photodetector 35, and performscontrol of the drive control unit 24 so that the light output level isheld constant. The drive control unit 24 controls the drive current ofthe laser diode 33.

The optical output signal from the photodetector 35, which contains thecommand signal CM, reaches the filter 37, at which only the commandsignal CM is extracted. The command contents of this command signal CMis decoded at the supervisory unit (SV) 38. The command contentsindicate which one of the temperature of the optical amplifier 11, theinput level, and the output level or other monitor data is to be output.The specified one monitor data MD is converted to a frequency signaloutput from a voltage-controlled oscillator (not shown in the drawing)in the supervisory unit 38 and applied to the drive control unit 24,thereafter reaching the wavelength-division multiplexer 32 as theresponse signal RS and modulating the optical carrier signal S.

In FIG. 14, the pair of optical amplifiers (AMP) for the upstream andthe downstream, which are in each of the optical amplifying repeaters 11shown in FIG. 13, are divided into the constitutional elements indicatedwith primes ('), to distinguish them from constitutional elements not somarked. Therefore, if a failure occurs at the point "X" in FIG. 14, inresponse to the command signal CM' transmitted from the lower right partof that drawing, the above-noted response signal RS is output (in themode which is the opposite of that in the example of FIG. 13).

There is additionally a monitor control mode, in which the commandsignal CM is sent from the end office A, the response signal RS beingsent as is from each of the optical amplifying repeaters 11, via theoptical transmission line 12 to the end office B, in which mode thealready presented explanation with regard to FIG. 14 can be applied asis.

Referring to FIG. 14, consider the case in which a cable failure occursat the point "X" in this drawing, causing the main optical signal Sinput to the optical amplifier 21 to be lost, representing a loss ofoptical input.

When such a loss of optical input occurs, because there is no longer anyinput of the main optical signal (carrier) to the optical amplifier 21,this optical amplifier 21 is no longer able to apply a modulation by theabove-noted response signal RS to this optical carrier signal. When thisoccurs, the optical amplifier 21 applies modulation by the RS signal tothe ASE (amplified spontaneous emission) instead of the main opticalsignal S. This ASE is a noise component of a relatively wide bandwidthwhich is inevitably generated from each optical amplifying repeater.

Although it is theoretically possible to apply a modulation inaccordance with the response signal RS to this ASE, in actuality themodulation of the amplified spontaneous emission by the response signalRS is insufficient. This can be explained in more detail as follows.

FIG. 15 is a graph which shows the relationship between the opticalamplifier 21 drive current and the ASE (amplified spontaneous emission)output. As shown in this drawing, in the range in which the drivecurrent I_(LD) of the optical amplifying repeater is small (smaller thanapproximately 250 mA), the variation of the amplified spontaneousemission with respect to the deviation in I_(LD) is relatively large.

In response to the loss of optical input, when the ALC (automatic levelcontrol) loop 22 operates sufficiently, so that drive current I_(LD) ofthe optical amplifier 21 becomes large, in this range in which I_(LD) islarge (larger than approximately 250 mA), the variation in the output ofthe amplified spontaneous emission with respect to the variation ofI_(LD) is relatively small. For this reason, when applying modulation tothe ASE by the amplitude of the normal response signal RS, the resultingmodulation output is extremely small, as indicated by p mW! in thedrawing. Ultimately, after the optical carrier signal S is lost, even ifmodulation is applied to the ASE by the response signal RS, it becomesdifficult to perform such modulation. As a result, it becomes difficultto receive the response signal RS at the end office and, in the worstcase, it becomes impossible to monitor the optical amplifying repeater,and impossible to perform troubleshooting of the fault point, thesebeing the problems cited previously.

In consideration of the above-noted problems, the present inventionenables the reception of the response signal at an end office, evenafter loss of the optical carrier signal due to a cable failure.

FIG. 1 is a drawing which shows the first basic configuration of anoptical amplifying repeater according to the present invention. In allof the drawings, similar constitutional elements are assigned the samereferences numerals or symbols.

(1) In the first aspect of the present invention, in an opticalamplifying repeater 11 which has an optical amplifier 21, an ALC(automatic level control) loop 22, which includes therein a drivecontrol unit 24 which drives and controls the optical amplifier 21 sothat the optical output therefrom is of a constant level, and asupervisory unit 23 is provided which applies a response signal RS, inresponse to a command signal CM from an end office, to the drive controlunit 24, a detection/control unit 41 is provided. When thisdetection/control unit 41 detects that the main optical signal S, i.e.,the optical input received from the optical transmission line 12, islost, it controls the drive control unit 24 so as to increase theamplitude of the response signal RS.

As will become clear from embodiments to be described later withreference made to various drawings, the present invention has thefollowing aspects in its embodiments.

(2) In the second aspect of the present invention, the detection/controlunit 41 comprises a filter and a comparator which detect, when the mainoptical signal S is lost, an increase of the output level of the ASE(amplified spontaneous emission) included in the optical output of theoptical amplifier 21, at a particular wavelength in the spectrum of theASE.

(3) In the third aspect of the present invention, the detection/controlunit 41 comprises a decoder which detects that the main optical signalsreceived from the optical transmission line 12 has been lost, thedetection is made from information, when it is returned from the endoffice, indicative of a drop of the modulation degree of the responsesignal RS received at that end office.

(4) In the fourth aspect of the present invention, the detection/controlunit 41 comprises a comparator which detects that the level of the mainoptical signal S has dropped.

(5) In the fifth aspect of the present invention, in an opticalamplifying repeater 11 which has an optical amplifier 21, an ALC(automatic level control) loop 22 including therein a drive control unit24 which drives and controls the optical amplifier 21 so that theoptical output therefrom is of a constant level, and a supervisory unit23 which applies a response signal RS, in response to a command signalCM from an end office, to the drive control unit 24, wherein an externalmodulator is provided at the output of the optical amplifier 21, whichapplies external modulation to the optical output from the opticalamplifier 21, and a detection/modulation unit is provided which makeswhen the loss of the main optical signal S is detected, the externalmodulator by modulation the response signal RS.

(6) In the sixth aspect of the present invention, thedetection/modulation unit comprises of a filter and a comparator, whichunit detects, when the main optical signal S is lost, an increase of theoutput level of the ASE (amplified spontaneous emission) included in theoptical output of the optical amplifier 21 at a particular wavelength inthe spectrum of the ASE.

(7) In the seventh aspect of the present invention, thedetection/modulation unit comprises a decoder which detects that themain optical signals receieved from the optical transmission line 12 hasbeen lost, the detection is made from information, when it is returnedfrom the end office, indicative of drop of the modulation degree of theresponse signal RS received at that end office.

(8) In the eight aspect of the present invention, thedetection/modulation unit comprises a comparator which detects that thelevel of the main optical signal S has dropped.

In the above-noted first aspect of the present invention, by means ofthe detection/control unit 41, when the loss of the main optical signalS is detected, the amplitude of the response signal RS is increased, sothat the modulation degree of signal RS to the ASE (amplifiedspontaneous emission) is increased.

In the above-noted second aspect of the present invention, when the mainoptical signal S is lost, the loss of the S signal is detected by theincrease in the ASE (amplified spontaneous emission) at a particularwavelength.

In the above-noted third aspect of the present invention, the loss ofthe main optical signal S is detected indirectly, by having the opticalamplifying repeater 11 notified of a drop in the modulation degree ofthe RS signal from the end office.

In the above-noted fourth aspect of the present invention, the loss ofthe main optical signal S is detected by directly monitoring the inputlevel of the main optical signal S.

In the above-noted fifth aspect of the present invention, an externalmodulator is employed to bring about the same effect as in theabove-noted first aspect of the present invention, in which themodulation degree is increased by increasing the amplitude of theresponse signal RS.

In the above-noted sixth through eight aspects of the present invention,the same effects are obtained as in the above-noted second throughfourth aspects of the present invention, respectively.

FIG. 2 is a drawing which shows an example of the first configuration ofa detection/control unit 41 according to the present invention. Adetection/control unit 41 according to this first configurationbasically comprises a bandpass filter (BPF) 42 and a comparator (COM)43. In addition, the reference numeral 44 denotes a photodetector (PD)which converts the optical signal from the filter 42 to an electricalsignal, and 45 is an optical coupler which splits off part of theoptical output from the optical amplifier 21. This first configurationwill be described with reference to related drawings.

FIG. 3 is a spectrum plot of the amplified spontaneous emission (ASE)which exists along with the main optical signal, and FIG. 4 is aspectrum plot of the amplified spontaneous emission (ASE) when the mainoptical signal is lost. The ASE output level when there is no cablefailure and the main optical signal S is present is, as shown in FIG. 3,approximately 0.16 μW at a particular wavelength (for example, 1528.8nm).

When a cable failure occurs, however, the ASE output level when there isno longer a main optical signal, as shown in FIG. 4, increases sharplyto approximately 16 μW at the above-noted particular wavelength.

The first configuration example is devised with consideration to theabove-noted fact, so that by setting the threshold level to the level thas shown in FIG. 4, when this threshold level th is exceeded, the lossof the main optical signal is detected, and control is performed so asto increase the response signal RS amplitude with respect to the drivecontrol unit 24 even more.

That is, the filter 42 shown in FIG. 2 extracts the ASE at theabove-noted particular wavelength (1528.88 nm). This extracted ASEsignal is converted to an electrical signal by the photodetector 44, andis input to the comparator 43. The comparator 43 has a reference voltageof V_(th), so that when this electrical signal has a level that exceedsV_(th), it is judged that the main optical signal S has been lost. Theabove-noted reference voltage V_(th) corresponds to the threshold levelth which is shown in FIG. 4. The output of this comparator 43 controlsthe drive control unit 24 so that the amplitude of the response signalRS increases.

FIG. 5 is a drawing which shows an example of the second configurationof a detection/control unit 41 according to the present invention. Inthis second configuration example, when for example a cable failureoccurs at the point "X" shown in this drawing, when information(indicated as CM' in the drawing) that indicates that the modulationdegree of response signal RS received at the end office B has dropped isreturned from that end office B, the detection/control unit 41indirectly detects the loss of the main optical signal from thisinformation CM'.

In addition, accompanying this detection, the drive control unit 24 iscontrolled so that amplitude of the response signal RS is furtherincreased.

Because the above-noted information CM', in a similar manner as thecommand signal CM, is represented as a pattern of "1" and "0" data, bydecoding this pattern of "1" and "0" data by means of the decoder (DEC)46, CM' is detected. The detected CM' is applied to the supervisory unit38.

FIG. 6 is a drawing which shows an example of the third configuration ofa detection/control unit 41 according to the present invention. In thisthird configuration example, the detection/control unit 41 monitorswhether or not the main optical signal S level at the input of theoptical amplifier 21 has dropped, and by means of this monitoringdetects the loss of the main optical signal when this drop in leveloccurs. When detection of this loss of signal is made, the drive controlunit 24 is controlled so that it increases the amplitude of the responsesignal RS.

Specifically, the detection/control unit 41 comprises a comparator (COM)47. To this comparator 47 is input the electrical signal resulting fromthe conversion, by means of the photodetector (PD) 49 of part of theoptical input which is split off at the input by the optical coupler 48.This input is the total optical power, which includes not only the mainoptical signal, but also the ASE. When the total optical power fallsbelow a reference level (reference voltage V_(th1)) which is establishedbeforehand, it is judged the main optical signal has been lost, and thecomparator 47 outputs a detection signal. This detection signal isapplied to the drive control unit 24.

FIG. 7 is a drawing which shows the second basic configuration of anoptical amplifying repeater according to the present invention.According to this second basic configuration, an external modulator 51and a detection/modulation unit 61 are provided. While thedetection/modulation unit 61 is similar to the already-describeddetection/control unit 41, when the loss of the main optical signal S isdetected, the detection/modulation unit 61, rather than applying theresponse signal RS at the drive control unit 24 to the optical amplifier11, applies it to the external modulator 51. By doing this, the responsesignal RS modulation, which is difficult to apply to the ASE, isperformed by an external modulator 11, thereby raising the modulationdegree to a level which enables reception at the end office.

In this manner, the external modulator 51 is provided at the output endof the optical amplifier 21, external modulation according to theresponse signal RS being applied to the output of the amplifier 21. Anexample of this is the widely known AOM (acousto-optic modulator), adevice which makes use of the acousto-optic effect.

The configuration of the detection/modulation unit 61 which performsmodulation control to the external modulator 51 is approximately thesame as already described with regard to the detection/control unit 41.

FIG. 8 is a drawing which shows an example of the first configuration ofa detection/modulation unit 61 according to the present invention. Inthe first configuration of the detection/modulation unit 61, theconfiguration is approximately the same as and the action is the same asthe first configuration example of the detection/control unit 41.Therefore, the reference numerals 72, 73, and 74 in this drawingcorrespond to the bandpass filter (BPF) 42, the comparator 43, and thephotodetector (PD) 44, respectively, which are shown in FIG. 2.

FIG. 9 is a drawing which shows an example of the second configurationof a detection/modulation unit 61 according to the present invention. Inthe second configuration of the detection/modulation unit 61, theconfiguration is approximately the same as and the action is the same asthe second configuration example of the detection/control unit 41.Therefore, the reference numeral 76 in this drawing corresponds to thedecoder 46 which is shown in FIG. 5.

FIG. 10 is a drawing which shows an example of the third configurationof a detection/modulation unit 61 according to the present invention. Inthe third configuration of the detection/modulation unit 61, theconfiguration is approximately the same as and the action is the same asthe third configuration example of the detection/control unit 41.Therefore, the reference numerals 77, 78, and 79 in this drawingcorrespond to the comparator (COM) 47, the optical coupler 48, and thephotodetector (PD) 49, respectively, which are shown in FIG. 6.

FIG. 11 is a drawing (1) which shows a detailed example of blocks 22 and23 in FIG. 1 and FIG. 7. FIG. 12 is a drawing (2), which is acontinuation of FIG. 11. In FIG. 11 and FIG. 12, the common parts ofFIG. 1 and FIG. 7 are grouped as one part, while the detection/controlunit 41 of FIG. 1 and both the external modulator 51 anddetection/modulation unit 61 of FIG. 7 are drawn separately on the samedrawing. Therefore, when making use of the detection/control unit 41,both the detection/modulation unit 61 and external modulator 51 areignored. In the same manner, when making use of both the externalmodulator 51 and the detection/modulation unit 61, the detection/controlunit 41 is ignored.

In FIG. 11 and FIG. 12, constitutional elements which are the same as inFIG. 1 and FIG. 7 are assigned the same reference numerals. However,with regard to the drive control unit 24, constituent elements thereofhave been assigned reference numerals such as 241, 242, and so on. Inthe same manner, with regard to the supervisory unit (SV) 38,constituent elements thereof have been assigned reference numerals suchas 381, 382, 383, and so on. Constituent elements which have a suffixedE (Emergency), such as 241E and 242E and so on, are redundant elementscorresponding to elements 241, 242, and so on.

The detection/control unit 41, which is an essential point of thepresent invention, operates so as to increase the gain of the laserdiode driver (LD-DRV) 241 when the main optical signal S is lost.

The detection/modulation unit 61, which is another essential point ofthe present invention, operates so as to apply the response signal RSfrom the laser diode driver 241 to the external modulator 51 when themain optical signal S is lost.

Each of the parts will be described below. The parts of FIG. 11 and FIG.12 marked IC are fabricated as an integrated circuit. This IC forms thecentral part of the optical amplifying repeater circuitry, whichperforms the ALC (automatic level control) and SV (supervisory control)functions.

In performing the ALC operation, first the laser diode drive currentlevel is established by the ALC amplifier (ALC-AMP) 36, whichestablishes the ALC level and frequency band, according to the voltagemonitored from the optical output of the optical amplifier 11, by meansof the photodetector (PD) 35. The laser diode driver (LD-DRV) 241 causesa power transistor 243 to operate and controls the laser diode pumpingdrive current so that the laser diode drive current is proportional tothat laser diode drive current level. Switching between the LD-DRV 241and 241E to perform switching between normal and standby, or between theworking and protection element is performed by a signal from the decoder(DEC) 384. The current control amplifier 245 controls a dummy currentwhich is the pumping laser diode drive current subtracted from aprescribed pumping laser diode drive current (maximum of 700 mA), bymeans of a power transistor 244. To achieve cancellation oftemperature-caused variations in the optical system, the referencevoltage of the ALC amplifier 36 (non-inverting input terminal) is variedin accordance with the temperature sensor (TEMP) 81, thereby providingtemperature compensation.

In performing the above-noted supervisory (SV) operation, a commandsignal CM (10-MHz sub-carrier) which has passed through a filter MCF(monolithic crystal filter) 37 is held constant, regardless of theoptical output, by having the supervisory amplifier (SV-AMP) 381 performgain control, and then demodulated to a baseband signal (a data patternof "1" and "0") by the detector (DET) 382, after which it is sent via aninterface (SV-INF) 383 to a decoder (DEC) 384. The detector (DET) 382outputs a command signal to the opposing line, and the supervisoryinterface (SV-INF) 383 receives a command signal from the opposing lineand sends it to the decoder (DEC) 384. At the decoder (DEC) 384,detection is performed for an address coincidence between thedemodulated command signal and the repeater identification signal ID,and if the address coincides, this is recognized as an accessing of thisrepeater 11, after which one of the monitor data MD is selected andoutput from an analog switch (ASW) 385. The monitor voltage (monitordata) is sent, via the ASW 385 to a voltage controlled oscillator (VCO)386 and then converted to a frequency and serves as the monitor signal.This monitor signal is controlled to an amplitude which is proportionalto the pumping laser diode drive current, by means of the modulators(MOD) 242 of both the home line and the opposing line, passes though thelaser driver (LD-DRV) 241, and is modulated onto the pumping laser diodedrive current.

A POWER-ON 83 resets, when the power is applied, the decoder (DEC) 384,places the repeater 11 into the normal state, selects the workingpumping laser diode, and shuts off the monitor output. The laser diodedeterioration detection unit 84 judges that the laser diode hasdeteriorated and switches to the spare laser diode 33E if the pumpinglaser diode drive current equals or exceeds a threshold value and alsothe backward light of the laser diode is equal to or lower than athreshold value. A PIN-MONI 82 monitors the normal state of thephotodiode 35, and generates a response signal RS, which indicates anabnormality thereof, if an abnormality occurs.

As described in detail above, the first through eighth aspects of thepresent invention have the following effects.

According to the first aspect of the present invention, it is possibleto continue to send the response signal RS to an end office even afterthe main optical signal S is lost.

According to the second aspect of the present invention, it is possibleto detect the loss of the main optical signal with high accuracy bymonitoring a signal at a particular wavelength of the ASE (amplifiedspontaneous emission).

According to the third aspect of the present invention, it is notnecessary to provide a mechanism in each optical amplifying repeater fordetecting a loss of the input signal.

According to the fourth aspect of the present invention, it is possiblefor each optical amplifying repeater to directly and reliably detect theloss of input.

According to the fifth aspect of the present invention, because anexternal modulator is used, there is a reliable improvement in themodulation degree of the response signal.

According to the sixth, seventh, and eight aspects of the presentinvention, the effects of the second, third, and fourth aspects,respectively, of the present invention are achieved.

We claim:
 1. An optical amplifying repeater comprising an opticalamplifier which is inserted into an optical transmission line andamplifies a received optical input, an ALC (automatic level control)loop including a drive control unit which produces an ALC signal tocontrol said optical amplifier, an optical output from said opticalamplifier being maintained at a constant level, and a supervising unitwhich produces a response signal RS, in response to a command signal CMfrom an end office disposed at one end of said optical transmissionline, and applies said response signal to said drive control unit, saidoptical amplifier being controlled, via said drive control unit, by bothsaid ALC signal and said response signal RS, further comprising adetection/control unit, loss of said received optical input from saidoptical transmission line is detected by said detection/control unit,said detection/control unit controls said drive control unit to increasethe amplitude of said response signal RS.
 2. An optical amplifyingrepeater according to claim 1, wherein said detection/control unitincludes a filter and a comparator, when said received optical input islost, an increase in output level at a particular wavelength in aspectrum of an amplified spontaneous emission (ASE) is detected by saidfilter and the detected increase is applied, via said comparator, tosaid drive control unit to increase the amplitude of said responsesignal R.S.
 3. An optical amplifying repeater according to claim 1,wherein said detection/control unit comprises a decoder (46) whichdetects that the main optical signals received from the opticaltransmission line 12 has been lost, the detection is made frominformation, when it is returned from the end office, indicative of adrop of the modulation degree of the response signal RS received at thatend office.
 4. An optical amplifying repeater according to claim 1,wherein said detection/control unit comprises a comparator which detectsa drop in the level of said main optical signal.